In recent years, fuel cell electric vehicles have been drawn attention from a viewpoint of environmental protection, such as for restricting an emission amount of carbon dioxide, which causes the global warming. The fuel cell electric vehicle is furnished with a fuel cell generating electricity by way of electrochemically reacting hydrogen (H2) with oxygen (O2) that is contained in the air so that electricity generated by the fuel cell is supplied to the driving motor to generate a driving force. Furthermore, hydrogen utilizing equipment wherein hydrogen is used as an energy source have been developed, besides fuel cells.
As a hydrogen supplying apparatus for hydrogen utilizing equipment including fuel cells, a fuel reforming system is known, wherein liquid raw fuel such as alcohols and hydrocarbons including methanol and gasoline is vaporized in a vaporizer, and the resulting gas is further reformed in a reformer to produce hydrogen-rich gas. This fuel reforming system has been drawn attention as an apparatus for supplying the fuel cell with fuel gas.
By the way, the vaporizer requires a heat source for vaporizing raw fuel. As a heat source for the vaporizer, for example, a catalyst combustor or a combustion burner is often provided in the fuel cell system for burning exhaust gas (off gas) from the fuel cell, namely, gas mixed by the remaining hydrogen-rich gas fed to the anode pole (hydrogen pole) of the fuel cell and the remaining air fed to the cathode pole (oxygen pole), in the presence of a catalyst.
In such a fuel cell system, when a constant load is applied (during the steady operation), combustion heat quantity for exhaust gas and heat quantity required for vaporizing raw fuel are kept balanced. In other words, the optimum amount of exhaust gas for vaporizing raw fuel is supplied to the catalyst combustor or the combustion burner. However, when the load is shifted from a low load (low power output) to a high load (high power output), increasing a supply of raw fuel does not instantly cause an increment in supply of exhaust gas to the catalyst combustor or the combustion burner because of the lead time due to system volume and the like. Accordingly, the increasing amount of the vapor content of the raw fuel does not instantly supplied to the catalyst combustor or the combustion burner as the increment in the exhaust gas. That is, a delay arises for increasing the introduction of fuel. As a result, since heat quantity supplied to the vaporizer becomes insufficient with respect to the heat quantity required for vaporizing raw fuel, it is impossible to produce in good response raw fuel vapor in accordance with the output.
With the foregoing drawback in view, the following raw fuel vaporizing apparatus with various countermeasures have been proposed.
(1) A raw fuel vaporizing apparatus such as shown in FIG. 12 provided with a heat accumulator in addition to a vaporizer so that the heat accumulator is previously heated to accumulate heat with the use of waste heat and the like, and this previously heated heat accumulator compensates lack of vaporizing heat quantity when the output of the fuel cell system is increased.
(2) A raw fuel vaporizing apparatus wherein heat capacity permitting an expected maximum load variation (from the minimum power output to the maximum power output) is applied to a vaporizer so that the heat capacity of the vaporizer compensates lack of vaporizing heat quantity when the output of the fuel cell system is increased.
(3) A raw fuel vaporizing apparatus such as shown in FIG. 13 provided with a line for supplying a catalyst combustor or a combustion burner, at which exhaust gas is converted into heat, with auxiliary fuel (for example, methanol) that is different from exhaust gas so that a supply of the auxiliary fuel compensates lack of vaporizing heat quantity when the output of the fuel cell system is increased.
However, in the aforementioned raw fuel vaporizing apparatus, the following drawbacks arise.
(1) In the case of employing the heat accumulator, after introducing raw fuel, since the temperature of the heat accumulator (the amount of accumulated heat) decreases with elapse of time, the amount of vapor production decreases, leading to unstableness in the amount of raw fuel vapor. Further, since the response in raw fuel vapor production mostly relies on the weight of the heat accumulator, considering the response seriously results in increased weight of the heat accumulator. As the result, since time during which the heat accumulator accumulates heat becomes longer, the raw fuel vaporizing apparatus is not suitable if the output frequently varies. Furthermore, when considering reduction in weight and size of the equipment, it is not preferable to provide a heat accumulator.
(2) In the case of increasing heat capacity of the vaporizer, the wider the output variation range is, the greater the heat capacity of the vaporizer will be. As the result, the following drawbacks arise.
a. The temperature within the vaporizer has to be retained at high temperatures, leading to restriction in use material and construction from a mechanical point of view as well as extended time for raising the temperature from a physical point of view. Therefore, this raw fuel vaporizing apparatus is not preferable if the output is varied frequently.
b. Increment in weight and size.
Further, since the temperature within the vaporizer (the amount of accumulated heat) decreases with elapse of time, the amount of vapor production decreases, leading to unstableness in the amount of raw fuel vapor.
Furthermore, once the temperature of the vaporizer decreases, it is difficult to raise the temperature to the original temperature. Therefore, if the output is varied frequently, this raw fuel vaporizing apparatus is not preferable.
Accordingly, it is not preferable to increase the heat capacity of the vaporizer thoughtlessly.
(3) In the case of supplying auxiliary fuel, since the heat used for burning auxiliary fuel with the heat capacity of the catalyst burner or the combustion burner is firstly spent for raising the temperature of the catalyst combustor or the combustion burner, all the heat does not instantly and effectively contribute to vaporizing raw fuel. Therefore, merely supplying auxiliary fuel is not sufficient to produce vapor in good response. Further, if the output is varied frequently, the operating efficiency of the raw fuel vaporizing apparatus decreases.
In view of the above, the present invention seeks to provide a raw fuel vaporizing apparatus, a method of vaporizing raw fuel, and a fuel cell system equipped with a raw fuel vaporizing apparatus, wherein raw fuel vapor is produced in good response and stably even when the output is varied from a low power output drive to a high power output drive.